This invention relates generally to antennas for radiating or receiving a circularly polarized wave and particularly relates to an antenna of this type consisting of a bifilar helix operating in the backfire mode.
For some applications it is desirable to provide an antenna for radiating a circularly polarized wave. Among circularly polarized antennas is a bifilar helix which has been used in the past. However, the conventional bifilar helix radiates in the forward direction. In other words, such an antenna may be considered a slow wave structure where the currents are in phase at each turn of the two helixes of the antenna. Hence the antenna must be fed at one end and the wave radiates in the opposite direction of the feed. If the antenna is to be used with a reflector such as a parabolic reflector it creates a mechanical problem for supporting the antenna. At the same time it is desirable to provide an antenna of high efficiency which completely illuminates the reflector without any spill-over of the radiation.
It is accordingly an object of the present invention to provide an antenna for radiating or receiving circularly polarized waves which is characterized by a high efficiency and low aperture blockage.
Another object of the present invention is to provide a bifilar helix which will provide good illumination of the reflector with low spill-over and which, due to its small diameter, provides a minimum of obstruction to the reflected wave.